After initially being selected for SELCON, do I go through the SELCON process every year? Do I get to serve until I reach my MRD?

Short answer is that in all likelihood, you'll be allowed to go to your retirement eligibility and then forced to retire.

I would be remiss if I didn't pile on (I assume) with your other leadership and verbally slap you upside your head for not only being non-educationally qualified for O-5 promotion because of not having completed your ILE-Common Core before the board, but for doing it twice! In your previous question* I wasn't aware that you took yourself out by not being educationally qualified for promotion.

Belated chastisement over. Having said that, to your questions (these are specific to a reserve component officer - RA officers are slightly different).

I referenced AR 600-8-29* (Officer Promotions) in your previous question because it lays out the specifics of being SELCON (specifically para 3-14). Your SELCON period varies and should have been specified in your notification.

GENERALLY, the SELCON period will depend on your retirement eligibility (i.e., 'getting your 20'). It could be 'indefinite', but in no case will you be allowed to continue beyond 24 years of commissioned service as an O-4. USUALLY your SELCON period will take you to your retirement eligibility if you're within six years of eligibility and USUALLY will be a few years if outside of that range (i.e., you were on the fast track because normally you'd be within six years of retirement eligibility when you go before your first board).

You won't go before another SELCON board during that period, however you do continue to go before the promotion board each year. If you're not promoted, retired, or seperated for something else before that period expires, then you'll be separated on the end date of that period (unless you're within two years of retirement eligibility).

Again, the specifics on the period of your SELCON should have been included with your notification. Make sure you look at the top of the notification as sometimes it's not spelled out in the body of the message and has some easily looked over item (i.e., "E: 1 JUN 2025" or something like that). If there isn't a period specified, then you'll have to contact the POC to verify that it is "until you his 24 years commissioned service" or if for some reason it was not included.
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* https://www.rallypoint.com/answers/recommended-for-selcon-how-long-do-i-have
* AR 600-8-29 - https://armypubs.army.mil/epubs/DR_pubs/DR_a/ARN30301-AR_600-8-29-000-WEB-1.pdf

I know someone who was a major on active duty who was selcon and stayed a major for 10 years and did eventually get selected for LTC. He was Number 1 on the promotion list

This HQDA Policy is clear as mud to me....but your answer may be found in Appendix C

https://armypubs.army.mil/epubs/DR_pubs/DR_a/ARN30242-HQDA_POLICY_NOTICE_600-2-000-WEB-1.pdf